The underlying causes of asthma and other allergic diseases remain poorly understood, even as the incidence of such diseases continues to increase. The "Hygiene Hypothesis" postulates that microbial exposure plays a key role in the development of allergic responses and that the rising incidence of such diseases in the developed world reflects population-wide changes in microbial exposure. Initially based upon epidemiological studies, the "Hygiene Hypothesis" is now at the convergence of many diverse areas within immunology and microbiology. Recent advances in our understanding of molecular mechanisms regulating innate and acquired immune responses challenge the Th1/Th2 explanation for the "Hygiene Hypothesis" favoring a broader "counter-regulatory" model. To date, no meeting has taken advantage of this recent explosion in knowledge. The goal of this meeting is to draw together investigators from multiple disciplines doing work relevant to understanding the environmental factors and mechanisms that affect the development of asthma and allergy. A range of key topics will include recent major advances in: 1) understanding how recognition of pathogens by the innate immune system determines the development of allergic (Th2) or inflammatory (Th1) responses; 2) understanding the relationship of susceptibility genes for asthma, allergy and anti-microbial responses; 3) development of allergic responses during infancy and childhood; 4) mechanisms of differentiation of Th2 cells and regulatory T cells, and 5) the application of microbial products in the treatment or prevention of allergy and asthma. By bringing together a critical mass of multidisciplinary researchers, this symposium should provide an ideal forum both for in-depth exposure of junior investigators (graduate students and post-docs) to a fast-moving, fruitful area of medical research, as well as enticing such scientists into the field. The fact that the symposium is being held in conjunction with one on "The Regulation of Mucosal Inflammation" is to the strong benefit of both.